Satellites orbiting the earth are used for various communication applications. Due to residual eccentricity and inclination of their orbits, even geostationary satellites change their position with respect to an earth station. Usually the movement is at a non-constant velocity.
When a ground-based transmitter sends a signal to a satellite, the phase of the received signal changes depending on the distance to the satellite and the movement of the satellite. The shift is a form of Doppler shifting, which effects the group delay measurement through a satellite communication channel.
An amplitude modulation or frequency modulation technique is widely used for group delay measurement. Phase modulation is also possible to measure group delay. The group delay (.tau..sub.g) is -.PHI./.OMEGA., where .PHI. is the modulation signal (envelope) output phase angle in radians and .OMEGA. is the modulation signal angular frequency in radians per second. The result is a group delay measurement in seconds of the communications channel modulation signal.
One problem with such a method for determining group delay is that the formula does not take into account that the path length from the earth station to the satellite is not constant during the measurement. For example, a range change of 15 meters may result in a measured group delay change of 100 ns from the actual value.
Another problem with such a method is that in most cases, a link analyzer is employed to obtain the data. Link analyzers are expensive and thus it is desirable to eliminate them if possible.
COMSAT Laboratories as proposed a method for measuring group delay. In this proposed method, a series of phase measurements are obtained and an offset frequency is found. The group delay measurements are then obtained. This set up is complicated, however, and, due to the many steps involved, the process is difficult to control. The measurements also are not applicable if the satellite moves at a non-constant velocity.